In conventional mowing machines the cutter bar consists of a steel bar of substantial thickness and width (eg. 12 mm thickness and 80 mm width) on which a plurality of teeth are bolted, these teeth being known as grass dividing teeth or sometimes as blade protection teeth, to form a unit known as the comb which is normally mounted rigid with the mowing machine frame.
The purpose of the comb teeth is to divide the grass or other vegetation to be mowed, and to provide a support for the cutting blade, which consists of a steel bar (eg. of 20 mm width and 6 mm thickness) on which a plurality of triangular or trapezoidal knives with sharp lower-surface edges are riveted so that they rest with sliding reciprocating movement on the upper surfaces of the teeth.
The teeth also carry an upwardly and rearwardly directed tang the purpose of which is to counteract the bending effect of the blade and the upper surface edges of the teeth in order to straighten the grass being out.
Teeth of this type can be considered as open anti-clog teeth because as the tang is inclined upwards in a rearward direction, the blade passage space between the upper surface of the teeth and the relative tang increases rearwards with the result that the grass entrained between the rear of the blade and the tang is readily expelled rearwards and therefore cannot cause clogging (ie cannot jam between the blade and teeth).
Conventional mowing machines have however the serious drawback that in the case of grass lying horizontally or tangled because of rain and wind, or if small lumps of organic or other material are encountered, the fact that the comb is fixed means that the comb teeth have difficulty in penetrating the grass to be cut, and this tends to be pushed forwards so that it accumulates on the point of the comb teeth so building up on the teeth and requiring momentary machine stoppages for the necessary comb cleaning. Conventional mowing machines therefore do not clog but instead suffer build-up, in addition to undergoing annoying and damaging vibration.
A problem well known to the expert of the art is that of bar vibration deriving from the reaction of the blade reciprocating motion and tending to extend to the frame. A well known solution to this problem is to connect the bar to the frame not rigidly but instead by articulated arms which together with the bar and frame form an articulated parallelogram. This solution was proposed for the first time in Italian Pat. No. 399,989. It was later improved as described in Italian Pat. Nos. 506,608 and 536,151.
In all machines incorporating this solution, the cutter bar is free to oscillate parallel to itself relative to the frame, this oscillation occurring by reaction to the reciprocating motion of the blade, so producing dynamic balancing of the mowing device to eliminate or at least considerably reduce the vibration transmitted to the frame. The bar oscillations have a much smaller amplitude than the blade stroke as the mass of the bar, for obvious mechanical requirements, is much greater than that of the blade, normally to the extent of 10:1 or more.
However these devices do not adequately solve a further problem, namely the build-up which occurs in conventional mowing machines. In this respect, as the comb mass is 10 or more times the blade mass, the oscillation amplitude of the comb is 10 or more times smaller than that of the blade and thus so small that in the case of grass horizontally lying or twisted due to wind and rain, the disturbance to the grass caused by the teeth of the comb which oscillates by reaction is not sufficient to keep the comb clean with the result that it blocks up and pushes the grass forwards so requiring frequent stopping for the necessary cleaning of the comb. Mowing machines of this type therefore do not clog or vibrate, but have the tendency to suffer build-up.
A further known method of dynamically balancing the mowing device and simultaneously solve the build-up and clogging problem is to use two blades which move in opposite directions, acting in the manner of shears. In this case the two blades, which are substantially identical and kept pressing against each other by suitable means, are supported and guided by a support bar rigidly connected to the frame, and in order to undergo opposing oscillation are operated by connecting rod-crank linkages or the like.
Such devices however do not produce an always acceptable cut because when the cutting edge of the two blades becomes rounded due to wear or when the means which press the blades together become slack, or again if any knives of the system deform due to impact or the like, the grass bends over between the two blades and becomes more torn than cut, as happens in the case of normal shears. Mowing machines of this type therefore do not suffer build-up, clogging or vibration, but undergo rapid deterioration of cutting quality and require costly maintenance.
To obviate these drawbacks, devices comprising a blade and counter-blade have been proposed in which a support bar of considerable thickness and width (eg. 80.times.12 mm) is provided and comprises guides in which the blade and counter-blade slide under oscillation, they being of substantially equal weight and driven by connecting rod-crank linkages or the like to produce opposing oscillatory motion of equal amplitude. Consequently, although the blade has a normal triangular or trapezoidal flat knife structure, the counter-blade or comb consists of two facing parallel elements spaced apart by a distance little greater than the thickness of the blade knives, so as to create a slit in which the blade knives slide in the manner of a guillotine.
One of the two elements of the counter-blade or comb, namely the lower one on which the blade knives slide with their lower sharp-edged surface, is sharpened to produce the cutting action in cooperation with said knives, whereas the other element only acts in a counteracting manner to straighten the grass being cut. The two counter-blade or comb elements have a plan shape similar to that of the comb teeth of conventional mowing machines and have their ends joined so that the slits in which the blade slides are closed at their front and rear. A device of this type is described for example in Spanish utility model 291,595.
Said devices of the blade and comb or double counter-blade type do not suffer build-up as the ends of the counter-blade undergo wide oscillatory movement and produce a good cut even if the knives are not properly sharpened or if there is a small gap between the sharpened parts of the blade and counter-blade, but have the serious drawback of undergoing clogging of the cutting system. In this respect, it is clear that as the blade knives are sharpened only on one face and the counter-blade is sharpened only on the element along which the knife cutting edges slide, no cutting action can take place between the opposite or rear face of the knives and that counter-blade element facing it. As the knife edges are of the stated wedge shape and taper from the top of their rear to the cutting edge, the grass is easily entrained in part between said rear and the facing element of the counter-blade and becomes tightly packed there, to produce clogging which results in a pressure which is sometimes so high as to deform the counter-blade elements. Even if such deformation does not occur, the relative movement of the blade and counter-blade is always strongly hindered by the frictional resistance produced by the clogging material, with consequent increased power requirement and greater mechanical stress in the motion transmission members. Mowing machines of this type consequently do not suffer build-up or vibration but clog to a serious extent.
In conclusion, no known mowing device is able simultaneously to provide dynamic balancing with wholly or partially elimination of vibration transmission to the frame, an always acceptable cut even if the knives are imperfectly sharpened and/or a gap exists between them and the elements on which they slide, and elimination of build-up in the comb and clogging of the cutting members.
An object of the present invention is to provide a mowing device which obviates all the aforesaid drawbacks while offering all the said advantages.
A further object of the invention is to provide a mowing device which minimizes the forces involved in transmitting motion to the cutting members, possesses maximum rigidity and results in minimum power consumption.
A further object of the invention is to provide a mowing device which operates regularly and reliably and in which working interruptions and maintenance are minimized.
A further object of the invention is to provide a mowing device which is highly efficient under all the said aspects, is of simple and robust construction, and is of low cost.
Further objects and advantages of the invention will be apparent from the description of a preferred embodiment given hereinafter.